In a typical color camera image sensor, three individual pixels are employed to form a single color pixel, with each individual pixel having either a red, green, or blue transmission filter placed over it so as to sensitize the pixel to the color its filter passes and desensitize it to the colors its filter blocks. The underlying semiconductor light sensor is typically a photovoltaic sensor (i.e., a photodiode) or a charge coupled device (CCD).
While such image sensors are widely used, they have several shortcomings. First, because three “basic” pixels are required to make one color pixel, such image sensors require a large chip area. For example, a 1 megapixel color camera image sensor requires a 3 megapixel “black and white” image sensor. Also, since a color pixel is actually a small array and not a “point”, such image sensors are sensitive to various imaging artifacts, such as “herringbone” effects, when imaging a fine regular pattern. Furthermore, the transmission filters are typically organic (e.g., plastic) dye-based and are not very temperature stable, are prone to bleaching, and are expensive.
In attempts to overcome these shortcomings, one color image sensor employs a vertically oriented stack of three separate photodiodes to form a single color pixel, with each of the three photodiodes consisting of p-n junctions formed from p- and n-doped semiconductor materials and each absorbing either red, green, or blue light. Because of the vertical nature of the pixel, both the size of the image sensor and imaging artifacts associated with differential absorption of light are substantially reduced. Additionally, because filtering is performed by the semiconductor materials forming the photodiodes, transmission filters are not required.
However, the operation of p-n junctions are very sensitive to defects in the crystalline structure of the semiconductors, which can cause electron-hole pairs formed by the absorption of incident light to recombine and generate heat rather than the desired photovoltaic effect. As such, formation of such photodiodes must be carefully controlled and, for best results, generally requires the use of single crystalline semiconductors. Additionally, formation of p-n junctions generally occurs at relatively high temperatures using diffusion or ion-implantation processes and are often sensitive of the type of substrate employed. Depending on the depth of the p-n junctions, epitaxial processes may also be required. As a result of the typically high temperatures required to form the p-n junctions, certain materials (e.g., glass and plastic) may not be used as the substrate material. Furthermore, certain semiconductor materials are not available in either p-type or n-type form and, thus, cannot be employed for use in photodiodes.